It resulted with downloading attachment (i.e. from S3) on each
instance validation even when attachment didn't change at all.

Allows a lambda to be passed to decide what format a style should
be processed as

Travis build: retry bundle install on e.g. timeout

There are some file types that the file command doesn't understand.
Notably, files that have a format, but are ASCII-text based (like ruby,
certificates, etc), show up as text/plain. This presents a problem,
because the mime-types gem either doesn't report them at all (as for
.pem files) or reports them differently (as for .rb files).

This change allows a Paperclip.options[:content_type_mapping] hash,
which allows for a mapping of { :pem => "text/plain" } which tells the
spoof detector that this is an OK matchup and to allow it.

It is now a requirement for attachments to do one of three things:
  1. Have a content_type validation (e.g. "image/*")
  2. Have a filename validation (e.g. *.png, *.gif)
  3. Explicitly *not* have one of those validations

The intent is to make the default more secure, and you have to
explicitly reject the security of a validation in order to not have one.

Add documentation for `preserve_files` option

This option was added to `has_attached_file` some time ago, see https://github.com/thoughtbot/paperclip/commit/65e8d4f6de50732d8e1b.

fixes indentation in code example

Just saw this indentation in passing. Thanks for Paperclip!